This invention relates to the field of data transmission, and in particular to a modulation scheme for transmitting data over a communication channel, for example, in a discrete multi-tone modulation system.
In order to increase the efficiency of data transfer over a communications channel, the data is transferred as symbols, each representing a number of bits. For example, in a QAM (Quadrature amplitude modulation) system, the symbols are represented by the amplitude and phase of the signals. Sixteen unique symbols, i.e. combinations of amplitude and phase, for example, will represent four bits at time. The symbols form a constellation of points on a phase amplitude diagram. As the number of symbols increases, so does the possibility for transmission errors. Forward error coding schemes are employed to permit the receiver to detect errors and recover the correct transmitted symbol.
A preferred form of coding in data communications is convolutional coding, which is a bit level encoding scheme which depends on the preceding bit sequence. In Trellis coded modulation, the number of symbols is increased to provide redundancy. Only certain transitions are allowed. In the event of an error, the receiver can detect the most likely correct transition with a knowledge of all possible allowed transitions.
Unlike block codes, which send data in predetermined blocks, convolutional codes do not cope well with burst errors. Partly, in answer to this problem, turbo codes have been developed. In essence, turbo code consists of two or more convolutional constituent codes separated by an interleaver acting on the input sequence of the first encoder. See for example, xe2x80x9cApplication of Turbo Codes for Discrete Multi-Tone Modulationxe2x80x9d, Hamid R. Sadjapour, ATandT Shannon Labs., 1996.
Turbo code is attracting more and more interest due to its larger coding gain. In a DSL (Digital Subscriber Line) system, turbo code has been used to replace trellis code to get better Bit-Error Rate (BER) performance. However, when the constellation size increases, the coding gain advantage of turbo code starts to fall odd. This is because the redundant bits make the constellation size even larger.
An object of this invention is to increase the data transmission rate, for example, in a DMT system.
According to the present invention there is provided a method of transmitting data over a communications channel, comprising receiving an incoming bit stream, passing at least some of said bits through a turbo encoder to generate turbo encoded output bits, and generating words corresponding to symbol points on a constellation in a trellis code modulation scheme using at least said bits passed through said turbo encoder.
In this invention, the turbo coder is preferably used to code only the least significant bit (LSB) in the constellation since the LSB is most sensitive to errors. The achievable data rate by this means is only a couple of dB away from Shannon capacity. The invention combines powerful turbo code with a trellis-coded modulation scheme to increase the data rate, preferably in a DMT (Discrete Multi-tone) system.
In a DMT system, multi-subchannels are used to transmit data, each with different carriers and different QAM constellations containing different numbers of bits per constellation point. Normally, the number of bits at each constellation point is an integer and a subchannel is unusable if it cannot support one data bit. In accordance with the invention, a spread spectrum algorithm can be combined with turbo trellis-coded modulation so that channels which carry less than one bit of information can also be used. As a result, the overall channel capacity can be increased greatly.